In recent years, recording by means of an ink jet recording apparatus has been vigorously applied to various printers, copying machines and facsimiles owing to its low noise and ease with respect to speeding up and coloration.
Such an ink jet recording method is a printing method wherein small droplets of an ink are ejected and deposited onto a recording medium.
The ink for the ink jet recording method contains a recording material (dye or pigment) and a liquid medium for dissolving or dispersing it as fundamental components and, if necessary, various additives.
An ink jet white ink provides a recorded product with a good visibility when printed on a surface with a low lightness such as a black surface. In addition, the white ink is also useful for marking industrial products such as those made of plastic products and are also suitable for printing onto woods, metals, glass, porcelain and leather, thus having been investigated from various aspects.
However, conventional white inks suffer sedimentation and aggregation of a pigment due to the difference in specific gravity between pigment particles and a liquid medium, and hence they involve the problem that fine nozzles of the ink jet head are clogged or that they are poor in storage stability.
In addition, there has been the problem that hiding power is insufficient due to the small size of pigment particles.
In order to prevent sedimentation and re-aggregation of pigments, various investigations have been made on the ink. For example, Japanese Patent Laid-Open No. 118767/1985 discloses a technique of adding a polymer containing acrylic ester monomer units for stabilizing dispersion of a pigment, and Japanese Patent Laid-Open No. 145570/1994 proposes to use an ink containing a pigment, a polymeric dispersant, water and a water-insoluble resin emulsion to provide an ink excellent in preventing clogging and in storage stability.
However, in order to acquire a sufficient dispersibility of the pigment, a large amount of the resin emulsion must be added to the ink and, as a result, there may result in a low printed density or blur, thus sufficient printing quality having been difficult to obtain.
As a technology to maintain dispersion stability of a pigment, various methods of surface-treating the raw material titanium dioxide have also been proposed. For example, Japanese Patent Laid-Open No. 277566/1992 discloses that use of titanium dioxide pigment having been surface-treated with an organic phosphoric acid ester in a thermoplastic resin serves to enhance dispersibility of the titanium dioxide pigment.
However, since titanium dioxide having been surface-treated with an organic phosphoric acid ester has a poor surface polarity, though it is stable when dispersed in a resin, it is unstable when dispersed in water. Thus, in the case of using it in an ink jet ink, there results formation of a precipitate in the ink or clogging of nozzles.